Various techniques are known for exercising and toning the many skeletal muscles of the human body. People who exercise often target the abdominal muscles for a variety of practical or aesthetic reasons. Exercises such as sit-ups are effective at toning the rectus abdominis muscles of the abdomen. When contracted, these muscles can make the front of the abdomen rigid or cause flexion of the torso. A device for exercising these muscles is taught in U.S. Pat. No. 6,440,045 to Gaston.
While many exercises and implements have been created to develop the rectus abdominis muscles, relatively little attention has been given to exercising the internal and external oblique muscles which wrap the sides of the abdomen above each hip. For some individuals wanting to achieve a desired body shape, this part of the body can be especially difficult to tone. The oblique muscles serve to swivel the upper body with respect to the hips.
One device that is commonly found in gyms provides for a user to sit upright and couple their arms to a yoke that swivels in a vertical axis roughly aligned with the user's spine. The yoke is in turn coupled to an adjustable weight stack or the like so that the user can vary the force required to turn the yoke. This device is large and cumbersome. Furthermore, the use of weights for resistance to motion also introduces inertia, increasing the chance that misuse of the device may result in injury to muscles and other tissues. The inertial effects increase as greater weights are used for resistance. The effectiveness of this device varies from person to person. Some people may properly use the oblique muscles to rotate the yoke whereas other people may rely upon other muscles to coax the yoke around.
U.S. Pat. No. 6,126,581 to Hartsel teaches a piece of exercise equipment designed to be placed behind a person's head and to hold the person's arms in a position substantially above and behind the chest as exercises are performed. Exercise activities performed while lying on one's back, such as sit-ups or “bicycling”, and using the Hartsel invention are intended to strengthen abdominal muscles, including oblique abdominal muscles. To increase resistance, weights may be added to the Hartsel device. In accordance with Hartsel, increasing resistance involves increasing overall mass of the device and increasing the rotational inertia.